1. Field of the Invention
This disclosure relates to pixel cells for active matrix displays and more particularly to thin film transistor devices which employ field shields to improve performance.
2. Description of the Related Art
E-ink capsules react to voltages placed on them and are used for generating images. The capsule includes white and black ink (E-ink), which are reactive and move in accordance with the voltage applied to the capsule or cell. To change image content on an electrophoretic E-ink display, new image information is written for a certain amount of time (e.g., 500 ms-1000 ms). As the refresh rate of an active-matrix is usually higher, this results in addressing the same image content during a number of frames (e.g., at a frame rate of 50 Hz, 25 to 50 frames).
Referring to FIG. 1, a schematic drawing of the E-ink capsule 12 is illustratively shown. Addressing of E-ink from black to white, for example, requires pixel capacitor plates 10a and 10b to be charged to −15 V during a 500 ms to 1000 ms interval. During this time the white particles drift towards the top (common) electrode, while the black particles drift towards the bottom (active-matrix back plane) electrode. Switching to black requires a positive pixel voltage. At 0 V on the pixel capacitor the E-ink docs not switch.
Referring to FIG. 2, an equivalent circuit 20 of a pixel 22 in an active-matrix display is shown. A row electrode 24 forms a storage capacitor for the pixels in a next row. Active-matrix displays are driven a row-at-a-time. During one frame time, all the rows are sequentially selected by applying a voltage that changes thin film transistors (TFTs) 26 from a non-conducting state (+25V) to a conducting state (−25V). In this line selection time, the pixel capacitors 28 and 30 (i.e. the total capacitance at the drain side of the TFT) of the selected row are charged to the voltage supplied on a column electrode 32 (+/−15V, 0V). During the remaining frame time (i.e. the hold time), the other rows are addressed. The TFTs 26 are then in their non-conducting state and the charge on the pixel capacitors 28 and 30 must be retained. Between image updates the active-matrix is at rest at 0 V for the row and column electrodes, the pixel pads and the common electrode.
CDE is the capacitor 30 providing the display effect, Cst is the storage capacitor 28 and Cgd is the parasitic gate-drain capacitor (not shown) in the TFT 26. In the circuit 20, the previous row electrode forms the storage capacitor line.